Production methods of this type for plastic films are known. For instance, DE 43 24 970 A1 discloses a method for producing three-dimensionally structured surfaces with the aid of an embossing roll, the surface of the embossing roll representing the negative form of the surface to be embossed. This technique is used for the production of thermoplastic films as a substitute for natural surfaces with a highly complex structure, such as calf leather, buffalo leather, et cetera, which has fine pores and is provided with a complex grain structure. Here, the surface of a positive form is machined with the aid of laser engraving, the laser (beam) being controlled in the process by way of data which were previously obtained with the aid of optical or mechanical scanning of the original surface. A method of this type can be used firstly to produce an embossing roll directly and without the requirement of further intermediate steps. Secondly, a method of this type also permits the production of intermediate forms or intermediate rolls which correspondingly contain the positive or the negative of the surface structure of the original surface.
U.S. Pat. No. 5,886,317 discloses a method for engraving a pattern into the surface of a workpiece, in which the surface information in the form of electric control signals is likewise generated and is stored with the aid of optical or mechanical scanning of a surface of a pattern master, which surface information is then used to control the engraving laser. Here, in the region of the transitions or joints, the surface information which is obtained there from the pattern master is engraved multiple times one after another onto the workpiece as an identical pattern. It is also disclosed to engrave the same surface information multiple times and/or alternately in an inverse information sequence (that is, forward and backward), and also to apply it with a certain randomness. By way of methods of this type, the transitions become somewhat softer, but still remain visible.
These methods in the prior art are suitable for the previously customary production of films which are more or less single-color, with the exception of a somewhat slightly “cloudy” coloring or large-area color change, as exists between different functional regions in films of this type, for example in dashboard films between the region close to or below the windshield, which region is to be configured to be as low-reflective and dull-colored as possible, and the region in the footwell which is to appear as comfortable and bright-colored as possible.
Color changes of this type are produced nowadays by way of, for example, gravure printing methods, in which the film which is provided with a surface structure or an embossing is colored in a color printing unit with the aid of color impression rolls. Methods of this type are expensive and inflexible, since always only the same color transition can be produced by way of one roll. In addition, the printed image cannot be reproduced in a suitable quality without relatively great expenditure.
When work is carried out by way of a classic printing unit, a reproducible homogeneity can be produced only with difficulty, since the press setting (throwing on of the rolls) usually takes place indirectly via the setting of the press nip. If the thickness of the material fluctuates, more or less of the structure is provided with coloring depending on the pressure or the pressure distribution, and the printed image and therefore the color application and the color distribution vary.
Films with color transitions are produced nowadays in large-scale production for different vehicles. Some color separation lines which are of very striking configuration even have to be hidden behind a cover which separates the two color regions, on account of the unsatisfactory printed image.
All the known methods have disadvantages such that either color boundaries cannot be produced accurately, or that “flowing” color transitions from one color to the other require considerable outlay for multiple rotary printing and, in addition, produce a printed image which can have fluctuations during the production. Moreover, the observer often has the perception that structures of the embossed three-dimensional surface of films of this type and the coloring do not “fit together” and sees and feels, for example, immediately that artificial leather in no way achieves the visual and haptic impression of the original.
The subjective evaluation by way of the human eye is still an extremely precise way of assessing a structural surface, which up to now has not been capable of being replaced by way of automatic methods, which evaluation by way of the human eye clearly registers even very small changes in the appearance or color of the surface. Transitions or boundary regions, the formation of repeating patterns and moulette stripes are as clearly noticeable as colorings which seem to be “unnatural”. Moreover, the phenomenon exists that the human eye assesses a surface which is observed from a relatively great distance in a completely different way to an observation at a small distance. It can therefore be the case that, for instance, an artificial leather surface which is observed in detail and from a small distance appears homogeneous, whereas the same artificial leather surface is discerned as being bumpy, streaky and unnatural in color when observed from a distance of several meters.